This invention relates to a blend of ethylene vinyl alcohol copolymer and amorphous polyamide which provides thermoformable barrier layers with improved resistance to thermal and oxidative degradation.
Blends of ethylene vinyl alcohol polymers with polyamides in general are known, and have been used in packaging applications as barriers to inhibit the passage of atmospheric oxygen or other gases. European patent application 0 305 146 discloses a blend of about 70 to about 95 percent by weight of an ethylene vinyl alcohol copolymer having a copolymerized ethylene content of about 20 to about 60 mole percent and a degree of saponification of at least about 90%, and about 5 to about 30 percent by weight of an amorphous polyamide. The composition is useful as a barrier layer in thermoformed containers.
European patent application 0 309 095 discloses a blend of about 50 to about 95 weight percent of an amorphous polyamide and about 5 to about 50 weight percent of a vinyl alcohol polymer having a copolymerized ethylene content of 0 to about 60 mol percent and a degree of saponification of at least about 90%. The blends have oxygen barrier properties which are relatively independent of humidity. Packaging films, laminates, and containers prepared therefrom are disclosed.
Blends of ethylene vinyl alcohol ("EVOH") polymers with polyamides in general are known, and have been used in packaging applications as barriers to inhibit the passage of atmospheric oxygen or other gases.
Japanese patent application 53-49050 discloses a blend of ethylene vinyl alcohol copolymer (EVOH) with 5 to 40 weight percent polyamide. The EVOH contains 20-50 mole percent copolymerized ethylene, and is saponified at least 90%. The polyamides disclosed include a copolymer of hexamethylene diamine with isophthalic and terephthalic acids, in mole ratios of 100/0 to 50/50. The blend is formed into a film, which possesses gas barrier properties. The barrier performance of the film is purported not to decline even in highly humid atmospheres.
U.S. Pat. No. 3,726,034 discloses mixtures of 70-99% polyamide and up to 30% of a hydroxyl containing polyolefin. The polyamides consist of linear unbranched polymer chains containing no additional functional groups. Exemplified are blends of nylon 6 and EVOH.
Japanese patent application 53-49050 discloses a blend of EVOH with 5 to 40 weight percent polyamide. The polyamides include a copolymer of hexamethylene diamine with isophthalic and terephthalic acids, in mole ratios of 100/0 to 50/50. The blend is formed into a film, which is said to possess excellent gas barrier properties.
U.S. Pat. No. 4,079,850 discloses a multilayer blow molded container, which contains a layer which may be EVOH, polyamide, or various blends, providing gas barrier properties. The polyamides which are mentioned are nylon 6, nylon 66, and nylon 12.
U.S. Pat. No. 4,427,825 discloses a composition of matter useful for making films, of polyamide and 1-65% EVOH. Nylons with melting points greater than 175.degree. C. are preferred, such as nylon 11 or nylon 12.
U.S. Pat. No. 4,500,677 discloses a resin composition comprising a mixture of two EVOH resins and a polyamide resin. The ratio of the EVOH resins to the nylon resin can be between 95:5 and 5:95. Nylon 6, nylon 6,6 and other polyamides having linear alkylene groups are specifically mentioned.
In spite of the excellent barrier properties of ethylene vinyl alcohol polymers, their use in blends has been limited to some extent because of the sensitivity of such polymers to thermal and oxidative degradation. In some instances blends of EVOH with polyamides have inadequate stability to degradation and gel formation. It is often observed that the melt viscosity of EVOH resins, whether alone or in a blend, increases with time, and the rate of increase increases with temperature. It is presumed that this effect may result from impurities remaining from the manufacturing process or introduced during blending, including interactions between the EVOH and components introduced by blending. It is known, for example that acids catalyze crosslinking reactions in EVOH, strong mineral acids such as HCl being especially potent. It is also possible that spontaneous dehydroxylation occurs at elevated temperatures, leading to interchain linkages. The resulting increase in viscosity can eventually cause gelation or solidification of the molten polymer. Such behavior can result in metal surfaces of processing equipment becoming coated with a varnish-like layer of intractable, discolored, degraded polymer. Buildup of this layer can cause a gradual rise in torque required for extruder screws and, when exfoliated, the sporadic appearance of gel particles in the extruded product, particularly when regrind is included in the composition. Furthermore, when molten EVOH is exposed to oxygen, e.g. through air incursion at feed and vent ports of extruders, ethylene vinyl alcohol copolymer can darken and crosslink to yellow-brown gel. As a result of these problems, comparatively low melt processing temperatures (210.degree.-230.degree. C.) are normally recommended for processing EVOH polymers, and even then such problems can persist.
There has been much activity to find a way to increase the thermal and oxidative stability of EVOH. As one example, British appln. 2,182,334 discloses a vessel comprising a composition of EVOH and a propylene resin or a thermoplastic resin having in the main or side chain a carbonyl group. The resin mixture is stabilized against gelation, discoloration, and reduction of gas barrier properties by incorporation of a hydroxide or higher fatty acid salt of an alkaline earth metal and an antioxidant.
U.S. Pat. No. 4,795,781 discloses a resin composition of saponified ethylene/vinyl acetate copolymer containing terminal carboxyl (--COOH) and terminal amide (--COOR') groups, wherein the number (A) of the terminal carboxyl groups and the number (B) of terminal amide groups satisfies the relationship (B)/(A)+(B).times.100.gtoreq.5. The polyamide resin is obtained by N-substituted amide modification of the terminal carboxyl group of the polyamide. It is preferable that the amount of the --COOH groups not converted is not more than 50 microequivalents/g polymer. When the terminal carboxylic groups have been thus modified, troubles of gel formation and of viscosity increasing are avoided. Suitable polyamides include those prepared from a variety of dibasic acids, including terephthalic acid and isophthalic acid.
U.S. Pat. No. 4,747,744 discloses an improvement in pinhole resistance or impact resistance in a resin composition of a saponified ethylene vinyl acetate and a polyamide having terminal end modification leaving carboxylic end groups of 3.times.10.sup.-5 equivalent/g or less. During molding of films, etc., there will occur no gelling or other trouble which will make molding difficult, and the molded product has satisfactory gas barrier properties. Suitable polyamides include nylon 6, nylon 610, nylon 12, etc.
The present invention provides a composition which has the desirable formability properties of blends of amorphous polyamides and EVOH, without suffering degradation in stability otherwise characteristic of such blends.